Add batch PDF processing support

[jeisaacs]

Summary

Enable users to queue and process multiple PDF pairs in a single run so large interleaving jobs require fewer manual steps.

Why

• Reduces repetitive uploads when processing dozens of documents
• Keeps processing consistent by applying the same order/filename settings to every pair
• Lays groundwork for future automation hooks

Proposal

• Allow selecting multiple PDF pairs (or a packaged archive) from the UI
• Stream each pair to the backend for sequential processing with progress surfaced to the client
• Store each generated file with sensible naming derived from the batch metadata

Acceptance Criteria

• UI supports configuring multiple PDF pairs before starting processing
• Backend can process a batch without blocking subsequent requests
• Activity log clearly groups entries by batch job and individual file
• Errors surfaced per item without aborting the entire batch